Nobody predicted the 7.3-magnitude earthquake that devastated central Taiwan's Chichi on Sept. 21, 1999, leaving some 2,400 people dead, more than 20,000 injured and causing economic losses topping $20 billion. Though thousands of tremors rattle Taiwan each year because of its volatile tectonic real estate, no one saw the earthquake coming. What could have been a big but less destructive quake became one of the worst disasters ever to hit the island.

It's tragedies like the Chichi earthquake that send research vessels into the world's waters a few times per year to try to improve our understanding of how, when and why earthquakes hit. But the technology they use to do that  long submersible cannons called airguns  has drawn loud criticism since the 1990s from marine-mammal experts. They say firing airguns in certain waters and at certain times of the year can be extremely harmful to a long list of sea creatures, including dolphins, whales, porpoises, giant squids, crabs and sea turtles. The noise the guns generate causes some animals to flee their native habitat, says Naomi Rose, senior scientist with the Maryland office of Humane Society International (HSI). "Their use can cause stress to some animals as well  and you should never underestimate the harm that stress can cause." (See 10 species facing extinction.)

The debate moved into the waters off Taiwan this spring when experts went out to collect data to better understand why the Chichi quake happened. The Marcus G. Langseth, owned by the U.S. National Science Foundation (NSF) and operated by the Columbia University's Lamont-Doherty Earth Observatory (L-DEO), scouted underground formations for almost four months around Taiwan this spring, during which the crew popped its 36 airguns in the water every 20 or 60 seconds, depending on the instruments used to record the acoustic waves. Airguns, which are towed underwater at the back of the ship, cause loud, explosive sounds at a low frequency made when their pressurized air gets released into the water. The sound waves they generate are used to help build a picture of the rock structure beneath the seafloor, delineating fault lines, cracks or underwater volcanoes.

If scientists "understand the kind of setting in which these earthquakes happen," they can assess the risk of future ones, says Harm Van Avendonk, a research scientist with the University of Texas at Austin's Institute for Geophysics, who participated in the survey. The results can affect policies like helping inform better earthquake-proof building codes. "That's where the research is of great help," he says.

But a regional network of marine-mammal experts says the Langseth may have done more harm than good during its nearly four-month tour. The area where the ship was sailing is home to about 34 cetacean species, of which seven are fragile marine-mammal species, like the critically endangered Indo-Pacific humpback dolphin that lives along a 60-mile (100 km) stretch of Taiwan's west coast. "Seismic airguns are very loud, and under certain circumstances they can cause actual physical damage," says Rose. "When a species such as the humpback dolphin is already facing many threats and is hovering on the brink of extinction, adding to their risks by subjecting them to stress from airgun surveys could be the difference between making it and not making it," Rose says. Michael Jasny, senior policy analyst with the National Resources Defense Council, agrees. "Airguns fundamentally alter the fabric of life in the ocean and they do it at an enormous scale far beyond that of a ship monitor to judge," he says. Airguns have been developed during the lifespan of marine mammals that are alive today, so they "haven't evolved to handle the industrial noise we're putting into the ocean, which has doubled every decade for the last 50 years," Jasny adds. (See pictures of 10 new species found in the Mekong Delta.)

But scientists onboard the Langseth and other seismic vessels say they do take the impact on animals into account. American vessels are required by U.S. law to submit their itineraries to the National Marine Fisheries Service (NMFS), which in turn determines how long and where the vessels can operate. Several individuals and organizations raised concerns about the proposed study, including potential interruptions to breeding times and migration routes. NMFS then revised the authorized track lines in light of the comments. "We typically start six to 12 months before a cruise with discussions with [L-DEO] about our cruise plan and its impact on marine mammals," Van Avendonk says. "We make adjustments to the plan ahead of time to avoid operations that would significantly impact marine mammals."

During its survey, the Langseth had five approved observers on board to watch for marine mammals for 30 minutes before any airgun use. The operation shut down if any were spotted. They used passive acoustic monitoring (PAM) to detect vocalizing marine mammals in times of poor visual clarity. But Rose says that animals are often silent, and some "have high frequency vocalizations, which can only be detected when a PAM system is quite close." In other words, it would be too late to avoid airgun harm. Lee-Ann Ford, president and founder of Hong Kongbased Linking Individuals for Nature Conservation (LINC), says the sound of airgun explosions is 265 decibels at the source, and 110 decibels almost five miles away. The approximate hearing threshold for humans and marine mammals is 180 decibels, so "at [five miles] they're still being harmed," Ford says. (Read TIME's review of eco-documentary The Cove.)

Not all marine-mammal experts agree. For all the "hype on risks to whales from seismic research," the benefits get little attention, says William Lang, a marine-mammal expert and former program director for ocean sciences environmental operations at the NSF. "The risk to people for not pursuing this type of research is simply not part of the story," Lang says. In the past, explosives like dynamite were used to create seismic waves. Improvement of recording devices and computer analysis has further decreased the airgun sound intensity required for geological research. Airguns are "the best existing technology to produce the desired type of sound," he says. Lincoln Hollister, professor of geosciences at Princeton University, also rejects claims that airgun use is correlated with damage to marine life, like lesions to the ears and brains or mass beachings. "In recent years, environmental groups have become increasingly negative about the use of airguns, and efforts have greatly increased to monitor studies," Hollister says. "Tremendous damage has and is being done to the credibility of environmental groups" and, he adds, their demands are "seriously setting back university-based science."

Environmental and animal-rights groups acknowledge the value of airguns but insist researchers could do a better job mitigating any potential damage they're causing. They suggest avoiding sensitive habitats and having observers monitor during mealtimes as well. "They are scientists who care about the environment  so they should do everything above board and by the book, without any reluctance," Rose of HSI says. "It is highly likely that there is better technology out there." With so many suggestions coming from both sides, common scientific goals  creating lower-intensity devices that create sound waves, understanding geologic processes and preserving marine mammals' safety  may be getting drowned out.

The original version of this story misstated the approximate hearing threshold for humans and marine mammals.